1. Field of the Invention
The present invention is directed to a substrate, and more particularly to a substrate including wiring for transmitting a differential signal.
2. Description of Related Art
Numbers of LSIs are being used as electronic components constituting essential portions of electronic appliances such as computers and portable telephones. LSIs are used in electronic appliances by being mounted on a substrate.
In recent years, a transmission speed of signals transmitted between numbers of LSIs has been accelerated. Therefore, a differential signal is required for the accelerated transmission speed of signals between LSIs. For such a differential signal, a single-layer balance-wiring (also referred to as a “lateral balance-wiring”) and a tandem balance-wiring are known.
In the single-layer balance-wiring, a positive polarity signal and a negative polarity signal of the differential signal are formed on the same layer of the substrate. In the tandem balance-wiring, the positive polarity signal and the negative polarity signal of the differential signal are formed on a different layer of the substrate, and an internal layer of the substrate is inserted between the positive polarity signal and the negative polarity signal.
FIG. 5 is a sectional view schematically showing a configuration of a multilayer substrate 21 of a related art (a first example of the related art) including single-layer balance-wiring 14. FIG. 6 is a plan view schematically showing the configuration of the multilayer substrate 21.
Referring to FIGS. 5 and 6, a cover layer 102, a GND (ground) layer 103, a wiring layer 104 and a GND layer 106 are successively formed with an interlayer insulating film (referred to as an “interlayer film” below). A pair of signals of the LSI 101 reach to the same layer of the substrate 21 through a through hole 117. The pair of signals are transmitted from a pair of solder balls 118 of the LSI 101. The pair of signals are connected to a wiring 107a and a wiring 108a through the through hole 117.
As shown in FIG. 5, the wiring 107a and the wiring 108a are formed on the same layer (i.e., a wiring layer 104) of the substrate 21. The pair of signals are connected to a wiring 107b and a wiring 108b via the wiring 107a and the wiring 108a, respectively. The wiring 107a and the wiring 108a are formed under the LSI 101. The wiring 107b and the wiring 108b are formed as an outside part with respect to the LSI 101. An impedance of the differential signal is specified by a spacing between the pair of signals (wirings) and a width of the wiring. Reference symbols 109, 110, and 112 respectively denote interlayer thicknesses.
On the other hand, FIG. 7 is a sectional view schematically showing the configuration of a multilayer substrate 22 of the related art (a second example of the related art) including a tandem balance-wiring 15 and a tandem balance-wiring 16. FIG. 8 is a plan view schematically showing the configuration of the multilayer substrate 22.
Referring to FIGS. 7 and 8, a configuration of the interlayer of the substrate 22 is the same as the configuration shown in FIGS. 5 and 6. The pair of signals of the LSI 101 reach to the different layer of the substrate 22 through a through hole 117. The pair of signals are transmitted from the pair of solder balls 118 of the LSI 101. The pair of signals are connected to the wiring 107a and the wiring 108a through the through hole 117.
As shown in FIG. 7, the wiring 107a and the wiring 108a are formed on different layers (i.e., the wiring layer 104 and a wiring layer 105) of the substrate 22, respectively. The pair of signals are connected to a wiring 107b and a wiring 108b via the wiring 107a and the wiring 108a, respectively. The wiring 107a and the wiring 108a are formed under the LSI 101. The wiring 107b and the wiring 108b are formed as outside parts with respect to the LSI 101. The impedance of the differential signal is specified by an interlayer thickness 111 between the wiring 107b and the wiring 108b, and the width of wirings. Reference symbol 111 denotes an interlayer thickness.
The single-layer balance-wiring 14 and the tandem balance-wiring 15 and 16 will be described below. As shown in FIG. 9, with the single-layer balance-wiring 14, a pair of signal lines can be formed on the single-layer wiring layer 104 in a lateral direction (a horizontal direction) in the multilayer wiring board 21.
However, in the single-layer balance-wiring 14, the size of the substrate in the lateral direction is increased by the single-layer balance-wiring 14 when the number of pairs of the single-layer balance-wiring 14 is increased.
On the other hand, the tandem balance-wiring 15 and 16 include a higher wiring containability than the single-layer balance-wiring 14 because the pair of signal lines are formed in the vertical direction (thickness direction) of the multilayer substrate 22. Further, the tandem balance-wiring 15 and 16 also include stronger resistance to crosstalk noise from a pair of signals adjacent to the tandem balance-wiring 15 and 16 than the single-layer balance-wiring 14.
The related arts are disclosed in the below mentioned documents.
[Patent document 1] Japanese Patent Laid-Open No. 2001-053507
[Patent document 2] Japanese Patent Laid-Open No. 2002-057467
[Patent document 3] Japanese Patent Laid-Open No. 04-255287